Silicone rubbers physical properties

Table 29.5 Physical properties of general purpose silicone rubbers (Values determined at 20°C after curing for 24 hours at 250 C) ...

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Ghosh, A. and De, S.K. Dependence of Physical Properties and Processing Behavior of Blends of Silicone Rubber and Fluorombber on Blend Morphology. Rubber Chem. Technol. 77(5), 856-872, November/December 2004. 

The degree of moisture present affects the properties of the silicone rubber vulcanisate. Moisture levels also determine the ease with which the filler is incorporated into the silicone rubber. Low moisture levels improve the final physical properties but definitely detract from the incorporation speed of the silica filler. 

The chloride is used to manufacture silicones, tetramethyl lead and triptane (2,2,3 trimethylbutane). Lesser uses include the manufacture of butyl rubber, higher halogenated methanes, methyl cellulose, quaternary ammonium compounds, methyl mercaptan, methionine, fungicides and pesticides (primarily the Me-arsenate herbicides). Recently the chlorinated fluorocarbons have replaced CH3CI as high volume refrigerants and propellants (ref. 32) Tables 12 and 13 list the chemical and physical properties and potential numbers of workers exposed to the monohalomethanes. 

As in carbon-black-filled EPDM and NR rubbers, the physical network in silica-filled PDMS has a bimodal structure [61]. A loosely bound PDMS fraction has a high density of adsorption junctions and topological constraints. Extractable or free rubber does virtually not interact with the silica particles. It was found that the density of adsorption junctions and the strength of the adsorption interaction, which depends largely on the temperature and the type of silica surface, largely determine the modulus of elasticity and ultimate stress-strain properties of filled silicon rubbers [113]. 

Sequence distribution studies on several types of rubber by 13C-NMR technique have been reported. Some of the more recent reports include silicone rubbers [28-30], SBR [31], acrylonitrile-butadiene rubber (NBR) [32,33], polyurethane [34,35], polyepichlorohydrin [36], ethylene-norbonene [37] and ethylene-propylene rubber [4, 16, 25, 38-44]. The NMR studies on EPDM have been carried out extensively, because it is one of the important parameters, which control the physical properties of the elastomer. For example, ethylene sequence can influence the crystallisation kinetic and melting behaviour of the rubber [38]. 

Prior to the introduction of the LPS process, the low consistency liquid silicone rubber was not considered for use in fabricated parts because of the inadequate physical properties. Recent advancements in the low consistency silicone elastomer technology, however, have led to the development of high strength material. 

RTV. (room-temperature-vulcanizing). Rubbers that have good physical properties and electrical properties similar to silicone rubber. 

Implanted polymeric materials can also adsorb and absorb from the body various chemicals that could also effect the properties of the polymer. Lipids (triglycerides, fatty acids, cholesterol, etc.) could act as plasticizers for some polymers and change their physical properties. Lipid absorption has been suggested to increase the degradation of silicone rubbers in heart valves (13). but this does not appear to be a factor in nonvascular Implants. Poly(dimethylsiloxane) shows very little tensile strength loss after 17 months of implantation (16). Adsorbed proteins, or other materials, can modify the interactions of the body with the polymer this effect has been observed with various plasma proteins and with heparin in connection with blood compatibility. 

Hot-melt thermoplastic elastomer systems (23. 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form physical cross-links below their crystallization or vitrification temperature, and the system therefore has elastomeric properties. The moduli and low-temperature characteristics of these materials can be tailored to compare reasonably well with silicone rubbers at -40 C. However, they are limited in high-temperature applicability because of enhanced creep or flow due to softening. 

Organo-silica compounds are widely used in a range of industries such as printing, oil production, and mining as lubricants and polishing compounds. They also find domestic use in silicon rubber and aerosol sprays because of their special physical properties, such as viscosity and dielectric constant. These compounds are also recognized as poisons/inhibitors in different catalytic processes, such as in atmospheric scrubbers, automobile exhausts, flammable gas... 

The silicone polymer backbone is composed of Si-O-Si bonds. This bond is very strong and stable with a bond energy of 87 Kcal/ mole. The polymer can tolerate 250°C to 300°C without decomposing.— The fully compounded silicone sealant, when cured to a rubber, can withstand 200°C for sustained periods of time with no special additives and even higher temperatures with polymer modifications and/or heat stability additives.— The Sl-O-Si molecular structure is also transparent to U.V., so silicone sealants are virtually unaffected by weather. Samples of silicone sealants used in exterior construction applications have been tested after 20 years of actual performance. These samples exhibited essentially no change in physical properties or adhesion during that time period. 

Yang, J. and Hsiue, G. 1996. Synthesis of acrylic acid grafted silicone rubber via preirradiation graft copolymerization and its physical and dielectric properties. J Appl Polym Sci. 61 221-229. 

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